Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-walled carbon nanotubes (MWNTs), were discovered by Iijima in 1991 (Iijima, Nature, 1991, 354, 56). Subsequent to this discovery, single-walled carbon nanotubes (SWNTs), comprising a single graphene sheet rolled up on itself, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals (Iijima et al., Nature, 1993, 363, 603; and Bethune et al., Nature, 1993, 363, 605).
SWNTs have highly anisotropic mechanical properties, however, by processing fully integrated single-walled carbon nanotube composites into nanotube continuous fibers (NCFs), their highly directional properties can be more effectively exploited (Barrera, J. of Mater. 2000, 52, 38). Manipulating these nanoscopic materials into an aligned configuration can be accomplished more easily by processing the composites into fibers, allowing for better macroscopic handling of these nano-sized materials. In some cases, the SWNTs are used as nanoscale reinforcements in a polymer matrix in order to take advantage of their high elastic modulus (approaching 1 TPa) and tensile strengths (in the range 20-200 GPa for individual nanotubes) (Krishnan et al., Phys. Rev. B. 1998, 58, 14013). SWNTs are, however, more likely to be incorporated in the matrix as ropes or bundles of nanotubes, as a result of van der Waals forces that hold many entangled ropes together. These ropes or bundles are reported as having tensile strengths in the range of 15-52 GPa (Shenderova et al., Critical Revs Solid State Mater. Sci. 2002, 27, 227; Treacy et al., Nature 1996, 381, 678; Lourie et al., Phys. Rev. Lett. 1998, 81, 1638).
Polypropylene is a thermoplastic material that has excellent chemical resistance, and good mechanical properties with tensile strengths in the range of 30-38 MPa and tensile modulii ranging from 1.1-1.6 GPa for the bulk material (Hertzberg, R. W. Deformation and Fracture Mechanics of Engineering Materials. 4th Ed. Publ. John Wiley and Sons, 1996). A number of researchers, such as Kearns and Shambaugh (Kearns et al., J. Appl. Polym. Sci. 2002, 86, 2079), and Moore et al. (Moore et al., J. Appl. Polym. Sci. 2004, 93, 2926), have incorporated SWNTs into polypropylene matrices. Kearns and Shambaugh reported a 40% increase in fiber tensile strength for composites containing a 1 wt. % loading of SWNTs, while Moore et al. did not find any significant improvements in mechanical properties. These studies seem to indicate that efficient load transfer between the polymer matrix and the stronger, reinforcing SWNTs was not necessarily achieved.
In processing carbon nanotubes and a thermoplastic matrix into a fully integrated composite system, the chemically inert nature of each of these materials must be overcome in order to facilitate good interfacial adhesion, which in turn allows for better load transfer when a tensile load is applied to the system. Ineffective interfacial bonding, and sliding of individual nanotubes within nanotube ropes, will hamper load transfer from the matrix to the fiber, thereby limiting the amount of mechanical reinforcement that can be achieved in the composite (Ajayan et al., Adv. Mater. 2000, 12, 750).
As a result of the foregoing, a method for enhancing interfacial adhesion between the carbon nanotubes and the surrounding polymer matrix in such above-described composites, would be quite beneficial.